In recent years, harbors have become much crowded with vessels of a large size. Therefore, it has been required that tug boats should operate to move the vessel toward and away from the shore in a safe and rapid manner. For this reason, there have now been extensively used tug boats equipped with a Z-type propulsion apparatus which can easily vary the direction of propulsion over the range of 360 degrees. There has been proposed a rotation control system for controlling the rotation of a rotary housing mounting a propeller unit of such a Z-type propulsion apparatus which comprises a steering handle for commanding the rotary housing to rotate by a desired amount of angle. The steering handle is provided with a potentiometer for detecting the angular movement thereof (steering angle .theta.1). There is provided in the system another potentiometer for detecting the angular movement of the rotary housing (follow-up angle .theta.2). And a vector calculation circuit is provided for outputting a sinusoidal signal sin(.theta.1-.theta.2) to a servo-control circuit, the signal sin(.theta.1-.theta.2) representing a sine of the difference of angle between the steering angle .theta.1 and the follow-up angle .theta.2. The servo control circuit produces a servo signal from the sinusoidal signal sin(.theta.1-.theta.2) and feeds it to a servomotor which in turn controls a hydraulic circuit in accordance with its rotational movement. The hydraulic circuit controls a hydraulic motor, and the hydraulic motor rotates the rotary housing via a gear mechanism.
With the construction of the above-described conventional system however, the rotary housing rotates in the opposite direction to that of rotation of the steering handle when the difference of angle (.theta.1-.theta.2) exceeds 180.degree., since the sinusoidal signal sin(.theta.1-.theta.2), whose polarity is reversed when the difference of angle (.theta.1-.theta.2) exceeds 180.degree., is directly inputted to the servo control circuit. As shown in FIG. 2, when the steering handle is operated in such a manner that the steering angle .theta.1 is abruptly varied from 0.degree. to 180.degree. clockwise, the rotary housing rotates not clockwise but counterclockwise by the follow-up angle .theta.2, as indicated by a broken line in the same figure. As a result, the tug boat mounting this system turns in the opposite direction, i.e., counterclockwise. As is described above, the conventional system has a deficiency that the rotary housing rotates in the opposite direction when the difference of angle (.theta.1-.theta.2) exceeds 180.degree..
It is therefore an object of the present invention to provide a rotation control system for controlling the rotation of a rotary housing mounting a propeller unit of a Z-type propulsion apparatus in which the rotary housing can be rotated in the selected direction even when the rotary housing is commanded to rotate by more than 180.degree., thereby a safe and good steerability being obtained.
It is another object of the invention to provide such a system which is simple in construction and can be manufactured at lower costs.